Steam cracking is the major commercial process for the production of light olefins, especially ethene and propene. In typical steam cracking, the hydrocarbon feed is first preheated and mixed with dilution steam in the convection section of the furnace. After preheating in the convection section, the vapor feed/dilution steam mixture is rapidly heated in the radiant section to achieve the desired thermal cracking. After the desired degree of thermal cracking has been achieved in the radiant section, the furnace effluent is rapidly quenched in either an indirect heat exchanger or by the direct injection of a quench oil stream.
An undesirable byproduct of the cracking process is often the deposition of carbon deposits, commonly referred to as “coke,” on the inner surfaces of the radiant tubes of the furnace. Depending on the feedstock being cracked, coke may also be deposited in certain tubes in the convection section, or in the quench system of the furnace.
Steam cracking furnace performance is most frequently limited by the build of coke inside the radiant tubes. The coke acts as a thermal insulator resulting in increasing radiant tube metal temperatures (TMT's) as a run progresses. Once the TMT's approach or reach the design limit, the furnace needs to be decoked.
The coke inside the tubes also causes a hydraulic restriction and higher coil pressure drop. When the pressure drop becomes high enough, the resulting higher backpressure causes a loss-of-critical-flow at the critical flow nozzles located at the inlet of the radiant section. Once critical flow is lost for a particular tube, the flow rate through that tube will be reduced which results in a higher coking rate, which aggravates the hydraulic restriction further. This cycle continues rapidly and a decoke operation will be required.
It is desirable to reduce the radiant tube metal temperatures without a significant adverse impact on throughput and production.
One method for reducing the radiant TMT's is to raise the process duty of the furnace convection section so that the required radiant duty is lower for a given feed rate, S/HC, and conversion. Raising the duty of the convection section will of course raise the temperature of the process gas leaving the convection section. The temperature of the process gas leaving the convection section is referred to as the cross-over temperature (“XOT”, not to be confused with the term used in industry to denote radiant coil outlet temperature “COT”).
The perception in industry seems to be that the temperatures in the convection section need to be limited such that any significant thermal cracking does not occur therein. The concerns are likely based on the assumed drawbacks that thermal cracking in the convection section or cross-over is too unselective for ethene, and that it will cause coking inside the convection section tubes.